This invention relates generally to coatings technology. More particularly, it concerns the use of protective coatings which contain open holes axially aligned with open holes in a substrate.
Substrates which are fabricated in an industrial setting are usually subjected to a variety of processing steps. For example, metal substrates, after being cast, may undergo many procedures to achieve a final product, such as grinding, cold-working, cleaning, annealing, grit-blasting, further cleaning, and the like. There may be a variety of designed features on or in the substrate which are incorporated early on in processing, and which must be preserved through all subsequent fabrication steps.
Turbine engines provide a good illustration. The xe2x80x9csubstratexe2x80x9d may be a turbine blade, or may be a combustion chamber (combustor), for example. The parts are often made from high temperature metallic alloys, often referred to in the art as xe2x80x9csuperalloysxe2x80x9d. When turbines are used on aircraft, they are typically run at a temperature as high as possible, for maximum operating efficiency. Since high temperatures can damage the alloys used in the engine, a variety of approaches have been used to raise the operating temperature of the metal components. One approach calls for the incorporation of internal cooling channels in the component, through which cool air is forced during engine operation. Thus, the xe2x80x9cdesigned featurexe2x80x9d in this instance is a pattern of cooling holes which extend from one surface of the part to another. For example, the holes may extend from a cooler surface of a combustor to a xe2x80x9chotxe2x80x9d surface which is exposed to combustion temperatures of at least about 1200 C. The cooling holes are usually formed in the substrate by specialized laser-drilling techniques. Cooling air (usually provided by the engine""s compressor) is fed through the holes from the cooler side to the hot side of the combustor wall. As long as the holes remain clear, the rushing air will assist in lowering the temperature of the hot metal surface and preventing melting or other degradation of the component.
Another technique for protecting the metal parts and effectively raising the practical operating temperature of an aircraft engine involves the use of a thermal barrier coating (TBC). The TBC is usually ceramic-based. TBC systems frequently also include a bond coat which is placed between the ceramic coating and the substrate to improve adhesion. The use of TBC""s in conjunction with the battery of cooling holes is sometimes the most effective means for protecting an engine part. However, incorporation of both systems can be very difficult. For example, the cooling holes sometimes cannot be formed in the engine part after a TBC has been applied, since lasers usually cannot effectively penetrate both the ceramic material and the metal to form the pattern of holes.
If the cooling holes are formed prior to the application of the TBC system, they may become covered and at least partially obstructed when the TBC is applied. Complete removal of the ceramic-metal material from the holes can be very time-consuming and ineffective, if not impossible. Any obstruction of the holes during engine operation can interfere with the passage of cooling air, can waste compressor power, and can possibly lead to engine component damage due to overheating.
Even if a type of laser could satisfactorily penetrate the TBC, registration and alignment difficulties would remain. For example, there would be no suitable technique for ensuring that the hole being drilled through the TBC is properly aligned with the hole previously drilled in the substrate itself.
From this discussion, one can readily understand that new methods for protecting certain features on metal substrates during subsequent processing steps would be welcome in industry. Of particular interest in the area of turbine engines would be new methods for providing open holes which communicate through various coating layers on engine parts.
These new methods should protect the designed features, but should also allow the particular features to become fully exposed after the other processing steps are complete. Furthermore, the techniques should also be useful for repairing TBC systems while retaining open holes axially aligned with previously-formed open holes in the substrate.
Moreover, the techniques involved should be completely compatible with the other processing steps, and should not adversely affect the substrate. For example, the strength and integrity of a turbine engine part should be completely retained after the treatment to protect the cooling holes has been completed.
The needs discussed above have been met by the discoveries outlined herein. One embodiment of this invention is directed to a method for temporarily protecting at least one passage hole in a metal-based substrate from being obstructed by at least one coating applied over the substrate, comprising the following steps:
(a) covering the hole with a curable masking material which forms a protrusion over the hole;
(b) curing the masking material;
(c) applying at least one coating over the substrate and the masking material, wherein the coating does not substantially adhere to the protrusion; and then
(d) removing the masking material to uncover the passage hole.
As discussed previously, the substrate often includes a row or an array of passage holes. They are frequently cooling holes within turbine engine components. The curable masking material exhibits substantially non-Newtonian flow characteristics which make it especially suitable for forming protrusions of the proper size and shape on the coating-side of an engine part. The material can be thermoplastic or thermosetting, and is usually used in an admixture with at least one filler or other rheology-modifying additive.
Another embodiment of this invention relates to the curable masking material itself, which comprises an extrudable resin composition which is thermally stable at elevated temperaturesxe2x80x94usually up to a temperature of at least about 350 C. The material exhibits substantially non-Newtonian flow characteristics, e.g., those of a Bingham solid. It may be an epoxy or phenolic resin, for example, used in conjunction with at least one organic or inorganic filler like graphite or silica. When cured, the masking material ideally does not serve as an adhesion site for protective coatings which are subsequently applied. The masking material is easily removed from the substrate after any related processing operations have been completed.
Numerous other details regarding these and other embodiments of the present invention are provided below.